Inspection method and inspection apparatus

ABSTRACT

An inspection method and apparatus comprising, a step of reflecting linearly-polarized light having a predetermined wavelength using an non-polarizing beam splitter after transmitting the linearly-polarized light through a half-wave plate, irradiating a sample with the linearly-polarized light having a polarization plane of a predetermined angle, causing the light reflected by the sample to be incident to an image capturing sensor through a lens, the non-polarizing beam splitter, and an analyzer, and acquiring an optical image of a pattern formed on the sample; acquiring a plurality of optical images by changing an angle of the analyzer or the half-wave plate, and obtaining an angle of the analyzer or the half-wave plate such that a value of (σ/√A) becomes a minimum; and a step of inspecting whether a defect of the pattern exists, wherein the pattern is a repetitive pattern having a period at a resolution limit or less.

CROSS-REFERENCE TO THE RELATED APPLICATION

The present divisional application is based upon and claims the benefitof priority from U.S. application Ser. No. 14/177,546, filed Feb. 11,2014, in which the entire disclosure of the Japanese Patent ApplicationNo. 2013-029453, filed on Feb. 18, 2013 including specification, claims,drawings, and summary, on which the Convention priority of the parentapplication is based, are incorporated herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to an Inspection Method and InspectionApparatus.

BACKGROUND

With high integration and large capacity of a Large Scale Integration(LSI), a circuit dimension required for a semiconductor element becomesincreasingly narrow. Using an original image pattern (that is, a mask ora reticle, hereinafter collectively referred to as a mask), areduced-protection exposure apparatus called a stepper or a scannerexposes and transfers the pattern on a wafer to form a circuit, therebyproducing the semiconductor element.

It is necessary to improve a production yield for costly LSI production.At this point, a shape defect of the mask pattern can be cited as alarge factor that degrades the production yield.

On the other hand, there is a demand for pattern formation having a linewidth of tens nanometers in a contemporary typical logic device. Theshape defect of the mask pattern also becomes finer in such a situation.Because dimension accuracy of the mask is enhanced in order to absorbfluctuations of various process conditions, it is necessary to detectthe defect of the extremely small pattern in mask inspection. Therefore,high accuracy is required for an apparatus that evaluates the pattern ofa transfer mask used in the LSI production. For example, Japanese PatentNo. 4236825 discloses an inspection apparatus that can detect the finedefect on the mask.

Recently, as a technique for forming a fine pattern, nanoimprintlithography (NIL) has attracted attention. In this technique, a templatehaving a nanoscale microstructure is pressured on a specific resistformed on a wafer to form the fine circuit pattern on the resist.

In nanoimprint technology, in order to increase productivity, pluralduplicate templates (replica templates) are produced using a mastertemplate, that is, of an original plate, and the replica templates areused while attached to different nanoimprint apparatuses. It isnecessary that the replica template be produced so as to correspondprecisely to the master template. Therefore, it is necessary that notonly the pattern of the master template but also the pattern of thereplica template be evaluated with high accuracy.

Generally the mask is formed with a dimension four times the circuitdimension. The pattern is reduced and exposed onto a resist on the waferby a reduced projection exposure device, using the photo-mask, andthereafter, the circuit pattern is developed. On the other hand, in thenanoimprint lithography, the template is formed with magnification equalto the circuit dimension. Therefore, the shape defect in the pattern ofthe template has the large influence on the pattern transferred onto thewafer compared with the shape defect in the pattern of the mask.Accordingly, it is necessary to evaluate the pattern of the templatewith higher accuracy compared with the case that pattern of the mask isevaluated.

Recently, with the progress of the fine circuit patterns, the patterndimension is finer than a resolution of an optical system of a patternevaluation apparatus. For example, when a line width of the patternformed in the template is less than 50 nm, the pattern cannot beresolved by a light source of DUV (Deep UltraViolet radiation) lighthaving a wavelength of about 190 nm to about 200 nm. The optical systemis relatively easily constructed for the DUV light having the wavelengthof about 190 nm to about 200 nm. Therefore, the light source of an EB(Electron Beam) is used. However, unfortunately the light source of theEB is not suitable for quantity production because of low throughput.

There is a demand for an inspection apparatus that can accuratelyinspect the fine pattern without generating the throughput degradation.

There are various types of defect in the pattern. Among others, ashort-circuit defect in which the lines short-circuit to each other andan open-circuit defect in which the line is disconnected have thelargest influence on performance of the mask or template. FIG. 1illustrates an example of the short-circuit defect. The two adjacentlines are connected to each other in a region Al to generate theshort-circuit defect. FIG. 2 illustrates an example of the open-circuitdefect. The line is partially disconnected in a region A2.

On the other hand, for the defect in which the edge roughness increasesas seen in a region A3 in FIG. 3, the defect has a restrictive influenceon the performance of the mask or template.

Even though all defects can be detected, that is, defects that may causea problem and defects that will not cause a problem, the inspection canbe efficiently performed when only the defect that may cause a problemis detected. However, the short-circuit defect, the open-circuit defect,and edge roughness (shown in the region A3 in FIG. 3) are less than orequal to a resolution limit. In the case that the short-circuit defect,the open-circuit defect, and the edge roughness are mixed in arepetitive pattern having a period at the resolution limit or less,brightness and darkness caused by the defect, such as the short-circuitdefect and the open-circuit defect, which becomes a problem, andbrightness and darkness caused by the edge roughness are notdistinguished from each other in observation with the optical system.The same holds true for a bright-field image and a dark-field image.This is because the short-circuit defect, the open-circuit defect, andthe edge roughness are identical in size, namely, spread to the size ofan extent of the resolution limit in an optical image.

FIG. 4 schematically illustrates a line and space pattern. In FIG. 4, itis assumed that the pattern dimension is smaller than the resolutionlimit of the optical system. In a region B1 of FIG. 4, the line patternis partially lacks. In a region B2, the pattern edge roughnessincreases. The defects are clearly distinguished from each other on anactual substrate. However, the defects cannot be distinguished from eachother when observed through the optical system. This is because theoptical system acts as a spatial frequency filter that is defined by awavelength X of the light emitted from the light source and a numericalaperture NA. FIG. 5 illustrates an example in which the spatialfrequency filter is applied to the pattern in FIG. 4. The defect in theregion B1 and the defect in the region B2 are identical in size, and thedifference of the shape is hardly recognized. Accordingly, theshort-circuit defect which is less that the resolution limit isdifficult to be distinguished with the defect caused by edge roughnesswhich is also less than the resolution limit.

The present invention has been devised to solve the above problems. Anobject of the present invention is to provide an inspection apparatusthat can accurately inspect the fine pattern without generating thethroughput degradation, more particularly an inspection apparatus thatcan distinguish the defect to be detected from a defect that is not tobe detected.

Other challenges and advantages of the present invention are apparentfrom the following description.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an inspection methodcomprising, a step of reflecting linearly-polarized light having apredetermined wavelength emitted from a light source using annon-polarizing beam splitter after transmitting the linearly-polarizedlight through a half-wave plate, irradiating a sample that becomes aninspection target with the linearly-polarized light having apolarization plane of a predetermined angle, causing the light reflectedby the sample to be incident to an image capturing sensor through alens, the non-polarizing beam splitter, and an analyzer, in this order,and acquiring an optical image of a pattern formed on the sample, a stepof acquiring a plurality of optical images by changing an angle of theanalyzer or the half-wave plate, and obtaining an angle of the analyzeror the half-wave plate such that a value of (σ/√A), which is obtainedfrom a standard deviation σ of a gray level and an average gray level Ain the optical images, becomes a minimum, and a step of inspectingwhether a defect of the pattern exists with respect to the optical imagethat is acquired at the angle of the analyzer or the half-wave plate atwhich the value of (σ/√A) becomes the minimum, wherein the pattern is arepetitive pattern having a period at a resolution limit or less, theresolution limit being defined by a wavelength of the light source and anumerical aperture of the lens.

Further to this aspect of the present invention, an inspection method,wherein the pattern is a line and space pattern, and the angle of thehalf-wave plate is set such that a polarization direction of the lightwith which the sample is irradiated becomes any angle except an angle inthe ranges of −5 degrees to 5 degrees and 85 degrees to 95 degrees withrespect to a length direction of a line of the pattern.

Further to this aspect of the present invention, an inspection method,wherein the angle of the half-wave plate is set such that a polarizationdirection of the light with which the sample is irradiated is in therange of 40 degrees to 50 degrees.

According to another aspect of the present invention, an inspectionapparatus comprising, an illumination optical system including a lightsource that emits linearly-polarized light having a predeterminedwavelength, the illumination optical system reflecting thelinearly-polarized light emitted from the light source through ahalf-wave plate using an non-polarizing beam splitter, forming thelinearly-polarized light into the linearly-polarized light having apolarization plane of any angle except an angle in ranges of −5 degreesto 5 degrees and 85 degrees to 95 degrees with respect to a repetitivedirection of a repetitive pattern formed on a sample that becomes aninspection target, and illuminating the sample, an imaging opticalsystem including an image capturing sensor that obtains an optical imageof the pattern formed on the sample, the imaging optical systemtransmitting the light reflected by the sample through a lens, thenon-polarizing beam splitter, and an analyzer, and forming an image ofthe light on the image capturing sensor, an image processor that obtainsan average gray level and a standard deviation in every predeterminedunit region in the optical image, and a defect detector that detects adefect of the sample, wherein a resolution limit defined by a wavelengthof the light source and a numerical aperture of the lens is a value inwhich the pattern is not resolved, and the image processor obtains anangle of the analyzer or the half-wave plate from the plurality ofoptical images acquired by changing the angle of the analyzer or thehalf-wave plate such that a value of (σ/√A) obtained from a standarddeviation σ of a gray level and an average gray level A becomes aminimum.

Further to this aspect of the present invention, an inspectionapparatus, wherein the angle of the analyzer or the half-wave plate canarbitrarily be adjusted using a rotating unit, and the angle of theanalyzer or the half-wave plate is controlled such that the value of(σ/√A) acquired by the image processor becomes the minimum.

Further to this aspect of the present invention, an inspectionapparatus, comprising a storage unit in which information from the imageprocessor is stored, wherein, with respect to a substrate in which aplurality of patterns having different structures are formed or aplurality of substrates in which single patterns having differentstructures are formed, the image processor acquires the angle of theanalyzer or the half-wave plate in every structure of the pattern fromthe plurality of optical images acquired by changing the angle of theanalyzer or the half-wave plate such that the value of (σ/√A) obtainedfrom the standard deviation σ of the gray level and the average graylevel A becomes the minimum, and the image processor transmits theacquired-angle of the analyzer or the half-wave plate to the storageunit, and based on information stored in the storage unit, the angle ofthe analyzer or the half-wave plate is controlled to acquire the opticalimage of the pattern formed on the sample.

Further to this aspect of the present invention, an inspectionapparatus, wherein the angle of the analyzer or the half-wave plate canarbitrarily be adjusted using a rotating unit, and operation of therotating unit is controlled based on the information stored in thestorage unit.

According to another aspect of the present invention, an inspectionapparatus comprising, an illumination optical system including a lightsource that emits linearly-polarized light having a predeterminedwavelength, the illumination optical system reflecting thelinearly-polarized light emitted from the light source by a polarizingbeam splitter, transmitting the linearly-polarized light through ahalf-wave plate and a quarter-wave plate, forming the linearly-polarizedlight into elliptically-polarized light having a major axis in adirection except an angle in the ranges of −5 degrees to 5 degrees and85 degrees to 95 degrees with respect to a repetitive direction of arepetitive pattern formed on a sample that becomes an inspection target,and illuminating the sample, an imaging optical system including animage capturing sensor that obtains an optical image of the patternformed on the sample, the imaging optical system transmitting the lightreflected by the sample through a lens, the quarter-wave plate, thehalf-wave plate, and the non-polarizing beam splitter, and forming animage of the light on the image capturing sensor, an image processorthat obtains an average gray level and a standard deviation in everypredetermined unit region in the optical image, and a defect detectorthat detects a defect of the sample, wherein a resolution limit definedby a wavelength of the light source and a numerical aperture of the lensis a value in which the pattern is not resolved, and the image processorobtains an angle of the half-wave plate from a plurality of opticalimages acquired by changing the angle of the half-wave plate such that avalue of (σ/√A) obtained from a standard deviation σ of a gray level andan average gray level A becomes a minimum.

Further to this aspect of the present invention, an inspectionapparatus, wherein the angle of the half-wave plate can arbitrarily beadjusted using a rotating unit, and the angle of the analyzer and thehalf-wave plate are controlled such that the value of (σ/√A) acquired bythe image processor becomes the minimum.

Further to this aspect of the present invention, an inspectionapparatus, comprising a storage unit in which information from the imageprocessor is stored, wherein, with respect to a substrate in which aplurality of patterns having different structures are formed or aplurality of substrates in which single patterns having differentstructures are formed, the image processor acquires the angle of thehalf-wave plate in every structure of the pattern from the plurality ofoptical images acquired by changing the angle of the half-wave platesuch that the value of (σ/√A) obtained from the standard deviation σ ofthe gray level and the average gray level A becomes the minimum, and theimage processor transmits the acquired angle of the half-wave plate tothe storage unit, and based on information stored in the storage unit,the angle of the half-wave plate is controlled to acquire the opticalimage of the pattern formed on the sample.

Further to this aspect of the present invention, an inspectionapparatus, wherein the angle of the half-wave plate can arbitrarily beadjusted using a rotating unit, and operation of the rotating unit iscontrolled based on the information stored in the storage unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the short-circuit defect

FIG. 2 illustrates an example of the open-circuit defect

FIG. 3 illustrates an example of the defect in which the edge roughnessincreases as seen in a region A3.

FIG. 4 schematically illustrates a line and space pattern.

FIG. 5 illustrates an example in which the spatial frequency filter isapplied to the pattern in FIG. 4.

FIG. 6 illustrates the bright-field optical system.

FIG. 7 illustrates a modification of FIG. 6, in which a polarizationbeam splitter, a quarter-wave plate, and the half-wave plate are used.

FIG. 8 illustrates angle dependence on the analyzer with respect to theamplitude of the brightness and darkness caused by the edge roughness,the electric field amplitude of the scattering light, and the electricfield amplitude of the zero-order light.

FIG. 9 illustrates the angle dependence on the analyzer, in which theamplitude of the brightness and darkness caused by the short-circuitdefect or the open-circuit defect and the electric field amplitude ofthe scattering light are added to that of FIG. 8.

FIG. 10 is a configuration diagram of an inspection apparatus 100 of thepresent embodiment.

FIG. 11 is a view illustrating a procedure to acquire the optical imageof the pattern formed in the sample 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Many patterns formed on a semiconductor wafer are repetitive patternssuch as a line and space pattern, namely, regularly repeating patternshaving periodicity, the repetitive pattern is also used in a masterpattern and a daughter pattern in the nanoimprint lithography.

In the case that an image of the pattern in which a line width is lessthan 50 nm is formed with an optical system in which DUV (DeepUltraViolet radiation) light is used, even if a theoretical-limitationlens (numerical aperture NA=1) is used, the pattern can only be resolvedby using a liquid immersion technique. However, in the case that thepattern is a repetitive pattern, regularity is disturbed to change agray level of an optical image near a defect when edge roughnessincreases in a part of the pattern, or when the pattern is partiallylacking. Accordingly, a short-circuit defect, an open-circuit defect,and the defect caused by the edge roughness can be detected by comparingthe gray levels of elements. The change in gray level will be describedin detail below.

When being acquired with the optical system, an image of fineirregularity (hereinafter referred to as roughness) of a pattern edgelocated within a range of a dimension corresponding to a resolutionlimit of the optical system becomes a dull shape having the dimension ofalmost the resolution limit of the optical system without resolving anindividual irregularity shape. Because an amplitude and a frequency ofthe edge roughness are random, the pattern regularity is disturbed, andthe image of the edge roughness is acquired as bright and darkunevenness over a whole range of a sample.

In the case that the pattern is partially lacking, similarly, the imageof the fine irregularity is magnified to a size of almost the resolutionlimit of the optical system. That is, because the regularity isdisturbed by the lack of the pattern although the pattern is notresolved, a region near the defect has the gray level different from anaverage gray level of a surrounding region. The same holds true for thecase that the pattern is partially connected to the adjacent pattern.

Thus, the defect can be detected by checking the change in gray level,even if the repetitive pattern has a period that is less than or equalto the resolution of the optical system. However, as described above,the detected defects having the resolution limit or less, namely, theshort-circuit defect or open-circuit defect and the bright and darkunevenness caused by the edge roughness are difficult to bedistinguished from each other.

The inventor has devised the present invention by paying attention tothe fact that the large defects such as the short-circuit defect and theopen-circuit defect have a large influence on a polarization state ofillumination light compared with the small defects such as the defectcaused by the edge roughness. According to the present invention, bycontrolling the polarization state of the illumination light and acondition of a polarization control element of the optical systemforming the image of the light reflected from a substrate that becomesan inspection target, the bright and dark unevenness caused by the edgeroughness can be removed with the polarization control element toextract only a change in amplitude of the short-circuit defect oropen-circuit defect.

For example, for the short-circuit defect in FIG. 1, sensitivity for anelectric field component of the illumination light varies betweenvertical and horizontal directions by connection of lines adjacent toeach other. For example, when linearly-polarized light perpendicularlyincident to a substrate has a polarization direction of 45 degrees withrespect to a direction of a line and space edge, while a verticalcomponent and a horizontal component of the electric field of theincident light are equal to each other, the horizontal component of theelectric field of the reflected light is larger than the verticalcomponent due to the short-circuit defect. As a result, the polarizationdirection of the light reflected from the short-circuit defect isinclined in the direction orthogonal to the direction of the line andspace edge. For the open-circuit defect as seen in FIG. 2, thepolarization direction is inclined in the direction of the line andspace edge.

On the other hand, the defect caused by the edge roughness in FIG. 3 isnot the defect caused by lines connected to each other or the defectcaused by the line being disconnected. Because an irregularity size inthe edge roughness, even if identified as a defect, is finer than theshort-circuit defect and the open-circuit defect, there is a smalldifference in sensitivity between the vertical and horizontal directionsof the electric field component of the illumination light. Accordingly,for example, when the linearly-polarized light perpendicularly incidentto the substrate has the polarization direction of 45 degrees withrespect to the direction of the line and space edge, the polarizationdirection of the light scattered by the edge roughness becomes a valueclose to 45 degrees that is of the polarization direction of theincident light. However, because the polarization direction isinfluenced by a base pattern having a periodic repetition, thepolarization direction does not completely become 45 degrees, rather thepolarization direction has a value slightly deviated from 45 degrees.

The defect can be classified by taking advantage of the difference ofthe influence exerted on the polarization state of the illuminationlight. Specifically, the classification of the defect can be performedusing the optical system in FIG. 6.

Referring to FIG. 6, the line and space pattern is formed in a mask1001, that is, the inspection target. At this point, an angle of ahalf-wave plate 1003 is set such that the linearly-polarized light canbe obtained. The linearly-polarized light has a polarization plane inwhich a polarization direction of the light with which the mask 1001 isilluminated is any angle except an angle in the ranges of −5 degrees to5 degrees and 85 degrees to 95 degrees, preferably the angle is in therange of 40 degrees to 50 degrees, more preferably the angle is 45degrees with respect to a length direction of the line of the pattern.Therefore, the difference in sensitivity between a large defect such asa short-circuit defect and the open-circuit defect, and the small defectsuch as the defect caused by the edge roughness, can emerge with respectto the electric field component of the illumination light. When theillumination light has the polarization plane of 0 degrees or 90 degreeswith respect to the length direction of the line of the pattern, becausethe sensitivity of the illumination light is expressed as, not a changein polarization direction, but only a change in reflectivity, the brightand dark unevenness caused by the edge roughness is not completelyremoved.

Referring to FIG. 6, the light, which is emitted from a light source1002 and transmitted through a half-wave plate 1003, is reflected by anon-polarizing beam splitter 1004 to illuminate the mask 1001 through anobjective lens 1005. The light reflected by the mask 1001 is incident toan image sensor 1007 after being transmitted through the objective lens1005, the non-polarizing beam splitter 1004, and an analyzer 1006. As aresult, an optical image of the pattern formed on the mask 1001 can beobtained.

As illustrated in FIG. 6, the analyzer 1006 is disposed in the imagingoptical system, which allows only the light in the specific polarizationdirection to be extracted. Specifically, the incidence of the scatteringlight to the image sensor 1007 from the defect can almost completely beprevented by setting the direction of the analyzer 1006 to the directionorthogonal to the polarization direction of the light scattered by theedge roughness. On the other hand, the light scattered by theshort-circuit defect and the open-circuit defect is transmitted throughthe analyzer 1006, and is incident to the image sensor 1007 because thepolarization direction is inclined. Accordingly, the optical image inwhich the short-circuit defect and the open-circuit defect remain whilethe bright and dark unevenness caused by the edge roughness is removedcan be obtained.

FIG. 7 illustrates a modification of FIG. 6. In the example of FIG. 6, anon-polarizing beam splitter such as a half mirror is used. On the otherhand, in the example of FIG. 7, a polarization beam splitter, aquarter-wave plate, and the half-wave plate are used. Therefore, a lightquantity loss generated in the non-polarizing beam splitter can beavoided.

In FIG. 7, the line and space pattern is formed in a mask 3001, that is,the inspection target. The light emitted from a light source 3002 isreflected in the direction of 90 degrees by a polarization beam splitter3006, transmitted through a half-wave plate 3004 a and a quarter-waveplate 3004 b, and focused onto the mask 3001 by an objective lens 3005.

In this case, the angle of the optical axis of the quarter-wave plate3004 b is set such that the elliptically-polarized light is obtained.The elliptically-polarized light has a major axis that exists at anyangle except an angle in the ranges of −5 degrees to 5 degrees and 85degrees to 95 degrees, preferably an angle in the range of 40 degrees to50 degrees, more preferably an angle of 45 degrees with respect to thelength direction of the line of the pattern. Therefore, a longitudinalcomponent and a transverse component of the electric field of the lightwith which the mask 3001 is illuminated are equal to each other. Thatis, the difference in sensitivity between the large defect such as theshort-circuit defect and the open-circuit defect and the small defectsuch as the defect caused by the edge roughness can emerge with respectto the electric field component of the illumination light. When theelliptically-polarized light of the illumination light has a major axisdirection of 0 degrees or 90 degrees with respect to the lengthdirection of the line, because the sensitivity of the illumination lightis expressed as, not the change in polarization direction, but only thechange in reflectivity, the bright and dark unevenness caused by theedge roughness is not completely removed.

On the other hand, the half-wave plate 3004 a is used to adjust theangle of the polarization direction of the light in order that thescattering light caused by the edge roughness in which the polarizationis slightly inclined is removed by the polarization beam splitter 3006.

For example, it is assumed that the plane of light incidence withrespect to the polarization beam splitter 3006 is matched with thedirection of the repetitive pattern formed in the mask 3001,furthermore, it is assumed that the direction of the optical axis of thehalf-wave plate 3004 a is set to 22.5 degrees with respect to thedirection of the repetitive pattern, and that the direction of theoptical axis of the quarter-wave plate 3004 b is set to 45 degrees withrespect to the direction of the repetitive pattern. Assuming that thescattering light of the edge roughness retains the incidence polarizedstate, characteristics of the light with which the mask 3001 isilluminated and the light reflected by the mask 3001 are expressed asfollows. As mentioned below, (PBS) is the polarization beam splitter,(λ/2) is the half-wave plate, and (λ/4) is the quarter-wave plate.

(PBS)→0 degrees, linearly-polarized light →(λ/2)→45 degrees,linearly-polarized light→(λ/4)→45 degrees, linearly-polarizedlight→(reflection surface)→45 degrees, linearly-polarized light→(λ/4)→45degrees, linearly-polarized light→(λ/2)→0 degrees, linearly-polarizedlight→(PBS)

As indicated by the change in the characteristic of the light, thepolarization direction of the linearly-polarized light exiting from thepolarization beam splitter 3006 is identical to the direction of therepetitive pattern (that is, 0 degrees), and the polarization directionof the linearly-polarized light becomes 45 degrees with respect to thedirection of the repetitive pattern when being transmitted through thehalf-wave plate 3004 a. The characteristic is unchanged even if thelinearly-polarized light is transmitted through the quarter-wave plate3004 b, and the mask 3001 is illuminated with the linearly-polarizedlight. Then, the light reflected by the mask 3001 is transmitted throughthe quarter-wave plate 3004 b and transmitted through the half-waveplate 3004 a, and the polarization direction of the linearly-polarizedlight becomes identical to the direction of the repetitive pattern. Thelinearly-polarized light is then incident to the polarization beamsplitter 3006.

In the optical system of FIG. 7, the light incident to the polarizationbeam splitter 3006 is the linearly-polarized light having thepolarization direction identical to the direction of the repetitivepattern, and the light is not transmitted through but reflected by thepolarization beam splitter 3006. That is, in the case that thescattering light of the edge roughness retains the incidence polarizedstate, the scattering light is blocked by the polarization beam splitter3006, and does not reach an image capturing sensor 3007.

It is assumed that the plane of light incidence with respect to thepolarization beam splitter 3006 is matched with the direction of therepetitive pattern formed in the mask 3001, that the direction of theoptical axis of the half-wave plate 3004 a is set to 23 degrees withrespect to the direction of the repetitive pattern, and that thedirection of the optical axis of the quarter-wave plate 3004 b is set to45 degrees with respect to the direction of the repetitive pattern.Assuming that the scattering light of the edge roughness retains theincidence polarized state, the characteristics of the light with whichthe mask 3001 is illuminated and the light reflected by the mask 3001are expressed as follows.

(PBS)→0 degrees, linearly-polarized light→(λ/2)→46 degrees,linearly-polarized light→(λ/4)→45 degrees, elliptically-polarizedlight→(reflection surface)→45 degrees, elliptically-polarizedlight→(λ/4)→44 degrees, linearly-polarized light→(λ/2)→2 degrees,linearly-polarized light→(PBS)

In the embodiment, the polarization direction of the linearly-polarizedlight reflected by the polarization beam splitter 3006 is identical tothe direction of the repetitive pattern (that is, 0 degrees), and thepolarization direction of the linearly-polarized light becomes 46degrees with respect to the direction of the repetitive pattern whenbeing transmitted through the half-wave plate 3004 a. Then thelinearly-polarized light having the polarization direction of 46 degreesbecomes the elliptically-polarized light having the polarizationdirection of 45 degrees with respect to the direction of the repetitivepattern when being transmitted through the quarter-wave plate 3004 b.The light is reflected by the mask 3001 and transmitted through thequarter-wave plate 3004 b, and the light becomes the linearly-polarizedlight having the polarization direction of 44 degrees with respect tothe direction of the repetitive pattern. Then, the linearly-polarizedlight is transmitted through the half-wave plate 3004 a, and thepolarization direction of the linearly-polarized light becomes 2 degreeswith respect to the direction of the repetitive pattern. Thelinearly-polarized light is then incident to the polarization beamsplitter 3006.

As described above, when the linearly-polarized light perpendicularlyincident to the substrate has the polarization plane of 45 degrees withrespect to the optical axis, because the scattering light caused by theedge roughness is influenced by the base pattern having the periodicrepetition, the polarization direction does not completely become 45degrees, but has the value slightly deviating from 45 degrees. On theother hand, in this example, because the polarization direction of thelight returning to the polarization beam splitter 3006 can be rotatedaccording to the angle of λ/2, the polarization direction can become 45degrees by restoring the polarization direction of the scattering lightcaused by the slightly-inclined edge roughness to the original state.That is, when the scattering light caused by the edge roughness has thepolarization direction of 43 degrees, the scattering light is rotated by2 degrees to set the polarization direction to 45 degrees. Therefore,the scattering light caused by the edge roughness can be completelyprevented from being transmitted through the image capturing sensor3007. Thus, the optical image in which the reflected light caused by theedge roughness is removed can be obtained by adjusting the angle of thehalf-wave plate 3004 a.

FIGS. 6 and 7 illustrate the examples of the bright-field opticalsystem. However, the optical image the defect caused by the edgeroughness is removed can be obtained by the polarization even in adark-field illumination system or a transmission illumination system.

A specific method for finding a condition that removes the defect causedby the edge roughness will be described below. As described above, thedefect caused by the edge roughness can be removed from the opticalimage using the optical systems in FIGS. 6 and 7. However, in order toremove the defect caused by the edge roughness, it is necessary tocontrol the polarization state of the illumination light and thecondition of a polarization control element of the optical system thatforms the image of the light reflected from the inspection target.

Generally, in the mask or template that becomes the inspection target,many pieces of edge roughness exist over the whole surface while fewshort-circuit defects or open-circuit defects exist. For example, whenthe optical image having the region of 100 μm×100 μm is acquired, thereis only a small possibility that the short-circuit defect or theopen-circuit defect is included in the region, and there are few defectseven if the short-circuit defect or the open-circuit defect is includedin the region. That is, the brightness and darkness of the optical imagein the region is substantially caused by the edge roughness. This meansthat the condition that removes the defect caused by the edge roughnessis obtained by one optical image having the dimension of about 100μm×about 100 μm.

The change in gray level caused by the edge roughness in the opticalimage can be removed by controlling the polarization direction of thelight incident to the image sensor on the imaging optical system side.Specifically, by controlling the direction of the analyzer in theimaging optical system, a quantity of scattering light that is incidentto the image sensor due to the edge roughness can be changed to vary thebright and dark amplitude in the optical image.

The bright and dark amplitude in the optical image can be expressed by astandard deviation of the gray level in each pixel. For example, whenthe optical system has a pixel resolving power of 50 nm, the opticalimage having the region of 100 μm×100 μm is expressed by 4 millionpixels. That is, a specimen of 4 million gray levels is obtained fromthe one optical image.

For the dark-field illumination system, the standard deviation isobtained with respect to the sample, the obtained value is defined as anextent of the scattering light caused by the edge roughness, and thepolarization state on the imaging optical system side is adjusted suchthat the value becomes the minimum.

On the other hand, for the optical image in the bright-field opticalsystem, the extent of the brightness and darkness caused by the edgeroughness is influenced by zero-order light. The reason is as follows.Because the fine periodic pattern that is less than or equal to theresolution limit exists in the inspection target, the polarization stateof the zero-order light changes due to a phase-difference effect causedby structural birefringence. Therefore, the light quantity that becomesa base also changes when the analyzer or the half-wave plate is rotatedin order to remove the reflected light caused by the edge roughness.Because the bright-field image is a product of the electric fieldamplitude of the scattering light from the short-circuit defect, theopen-circuit defect, or the edge roughness and the electric fieldamplitude of the zero-order light, the extent of the brightness anddarkness caused by the edge roughness is influenced by the intensity ofthe zero-order light as described above.

FIG. 8 illustrates angle dependence on the analyzer with respect to theamplitude of the brightness and darkness caused by the edge roughness,the electric field amplitude of the scattering light, and the electricfield amplitude of the zero-order light. In the bright-field opticalsystem, when the polarization state of the scattering light caused bythe edge roughness is not matched with the polarization state of thezero-order light influenced by the structural birefringence, the angledependence that is the product of both on the analyzer with respect tothe amplitude of the brightness and darkness caused by the edgeroughness has two local minima as illustrated in FIG. 8.

The reason the amplitude of the brightness and darkness caused by theedge roughness is expressed by the product of the electric fieldamplitude of the reflecting light from the defect caused by the edgeroughness and the electric field amplitude of the zero-order light willbe described below.

It is assumed that the electric field amplitude of the zero-order lightis as follows.

E ₀ =f(θ)

The electric field amplitude of the scattering light caused by the edgeroughness is described as follows.

E _(r) =g(θ)

The zero-order light becomes elliptically-polarized light having a longaxis in a predetermined direction by the influence of the birefringencecaused by the fine pattern. Therefore, f(θ) has one local maximum in therange θ of 0 degrees to 180 degrees, and becomes a function having thelocal minimum greater than 0. On the other hand, in the scattering lightcaused by the edge roughness, because the roughness does not have theperiodicity, the phase difference is small, if any, and the linearpolarization is substantially maintained. Therefore, g(θ) has one localminimum in the range θ of 0 degrees to 180 degrees, and becomes afunction having the local minimum close to 0.

As indicated by the following equation, a signal intensity I of thebright-field image is expressed by interference between the electricfield of the zero-order light and the electric field of the scatteringlight caused by the edge roughness.

I=<|E ₀ exp{i(ωt)}+E _(r) exp{i(ω+φ)}|>=E ₀ ² +E _(r) ²+2E ₀ E _(r)cos(φ)

In the above equation, because E₀ ² is a square of the zero-order light,namely, a base light quantity I₀, an amplitude I_(r) of the brightnessand darkness caused by the edge roughness is expressed by the followingequation.

I _(r) =I−I ₀ =E _(r) ²+2E ₀ E _(r) cos(φ)

Where φ is the phase difference between the zero-order light and thescattering light, and depends on a focal position of the substrate. Forexample, the following equation is considered as the condition that thebrightness and darkness caused by the edge roughness become thestrongest.

cos(φ)=1

Even though the edge roughness is extremely fine, as described abovewith reference to FIG. 8, the electric field amplitude of the zero-orderlight does not become zero due to the influence of the birefringenceeven if the angle θ of the analyzer is changed and thus, is approximatedas follows.

E_(r)>>E₀

Accordingly, the amplitude of the brightness and darkness caused by theedge roughness can be simplified as expressed by the following equation.

I_(r)=2E₀E_(r)

Therefore, in the periodic pattern that is less than or equal to theresolution limit, the amplitude of the brightness and darkness caused bythe edge roughness is expressed by the product of the electric fieldamplitude of the zero-order light and the electric field amplitude ofthe scattering light caused by the edge roughness.

As described above, because the E₀ and E_(r) depend on the angle θ ofthe analyzer, I_(r) is expressed by the following equation.

I _(r)=2f(θ)g(θ)

Accordingly, in the case that the value of the angle θ at which thefunction f(θ) becomes the minimum differs from the value of the angle θat which the function g(θ) becomes the minimum, I_(r) that is of theproduct of f(θ) and g(θ) has the two local minima.

In order to remove the influence of the scattering light caused by theedge roughness and improve sensitivity for detecting the short-circuitdefect or the open-circuit defect, it is necessary to find, not thecondition that the function f(θ) caused by the zero-order light becomesthe minimum, but the condition that the function g(θ) caused by the edgeroughness becomes the minimum. This is because the minimum of thefunction f(θ) is only the condition that the base light quantity becomesthe minimum and therefore the influence of the edge roughness is hardlyremoved.

The condition that the function f(θ) becomes the minimum is obtained bya calculation using a standard deviation σ of the gray level of theoptical image and an average gray level A. The standard deviation σincludes various noise factors, and particularly the standard deviationσ is largely influenced by the brightness and darkness caused by theedge roughness. Therefore, the standard deviation σ can be regarded asthe following equation.

σ∝I _(r)=2f(θ)g(θ)

Because the average gray level A of the optical image is the base lightquantity, namely, the intensity of the zero-order light, the averagegray level A is expressed as follows.

A∝I ₀ =E ₀ ² =f(θ)²

Accordingly, the function g(θ) is obtained by the following equation.

g(θ)∝σ/f(θ)=σ/√{square root over (A)}

Thus, the electric field amplitude of the scattering light caused by theedge roughness is proportional to a value in which the standarddeviation σ of the optical image is divided by a square root of theaverage gray level A. In order to find the condition that minimizes theamplitude of the brightness and darkness caused by the edge roughness,the optical image is acquired while the angle θ of the analyzer isvaried, and the value in which the standard deviation of the gray levelin the acquired optical image is divided by the square root of theaverage gray level. Then the angle θ is obtained such that the valuebecomes the minimum.

FIG. 9 illustrates the angle dependence on the analyzer, in which theamplitude of the brightness and darkness caused by the short-circuitdefect or the open-circuit defect and the electric field amplitude ofthe scattering light are added to that of FIG. 8. As described above, inthe large defect such as the short-circuit defect and the open-circuitdefect, the vertical direction and the horizontal direction differ fromeach other in the sensitivity with respect to the electric fieldcomponent of the illumination light. Accordingly, when the electricfield amplitude of the scattering light caused by the large defectbecomes the minimum, the angle θ of the analyzer differs from that ofthe scattering light caused by the edge roughness. That is, even if theangle θ is applied when the electric field amplitude of the scatteringlight caused by the edge roughness becomes the minimum, the electricfield amplitude of the reflected light caused by the short-circuitdefect or the open-circuit defect does not become the minimum.Therefore, the short-circuit defect and the open-circuit defect can bedetected without being buried in the amplitude of the brightness anddarkness caused by the edge roughness.

When the electric field amplitude of the scattering light caused by theedge roughness becomes the minimum, the value of the angle θ depends ona structure of the pattern formed in the inspection target. For example,the value of the angle θ at which the electric field amplitude becomesthe minimum also changes when a pitch, a depth, or a line and spaceratio of the pattern changes.

The angle θ is obtained in every substrate that becomes the inspectiontarget when the electric field amplitude of the scattering light causedby the edge roughness becomes the minimum.

One example, for obtaining the value of the angle θ will be describedusing the inspection apparatus provided with the bright-field opticalsystem as shown in FIG. 6. First, before the inspection, the opticalimage of the substrate that becomes the inspection target is acquired,and the standard deviation σ of the gray level and the average graylevel A of the optical image are calculated. The condition that functiong (θ), which expresses the electric field amplitude of the reflectedlight caused by the edge roughness, becomes the minimum is obtained bythe calculation using the standard deviation σ of the gray level and theaverage gray level A of the optical image. That is, as expressed by thefollowing expression, the function g(θ) is proportional to a value inwhich the standard deviation σ of the gray level of the optical image isdivided by a square root of the average gray level A.

g(θ)∝σ/√{square root over (A)}

In order to find the condition that minimizes the bright and darkamplitude caused by the edge roughness, the value of (σ/√A) is obtainedfrom the angle θ of the analyzer and the average gray level A. The angleθ at which the value of (σ/√A) becomes the minimum, specifically theangle θ of the analyzer in the bright-field optical system isdetermined. Alternatively, the angle θ may be determined by setting theangle of the half-wave plate to θ.

In the case that the bright-field optical system includes a light sourcethat emits linearly-polarized light, a half-wave plate, annon-polarizing beam splitter, and an analyzer, while the half-wave plateis disposed between the light source and the non-polarizing beamsplitter, the angle of the half-wave plate is desirably set, such thatthe polarization direction of the light with which the substrate isilluminated, at 0 degrees or an angle near 0 degrees, and 90 degrees oran angle near 90 degrees with respect to the pattern formed on thesubstrate, specifically any angle except an angle in the ranges of −5degrees to 5 degrees and 85 degrees to 95 degrees.

On the other hand, for the inspection apparatus provided with thedark-field optical system, the optical image of the substrate thatbecomes the inspection target is acquired before the inspection, and thestandard deviation σ of the gray level of the optical image iscalculated. The angles of the wave plate and analyzer, which areprovided in the dark-field optical system, are adjusted such that thestandard deviation σ becomes the minimum.

In another method for obtaining the angle θ, the optimum angle θ ispreviously obtained in every structure of the pattern formed on thesubstrate that becomes the inspection target. Before the inspection, theangle θ at which the electric field amplitude of the scattering lightcaused by the edge roughness becomes the minimum, is obtained using asubstrate different from the inspection target. It is assumed thatplural patterns having different structures are formed on the surface ofthe substrate, and the angle θ is then obtained for every structure ofthe pattern. A table for showing the structure of the pattern and thecorresponding optimum angle θ is formed and stored in a storage unitprovided in the inspection apparatus, and the table suitable to thesubstrate that becomes the inspection target is used during theinspection.

One example, for obtaining the value of the angle θ will be describedusing the inspection apparatus provided with the bright-field opticalsystem as shown in FIG. 6. Using the substrate in which plural patternshaving different pattern pitches, pattern depths, or line and spaceratios are provided, an optical image of the substrate is acquired.Then, the standard deviation σ of the gray level and the average graylevel A of the acquired optical image are calculated in every structureof the pattern. The value of (σ/√A) is obtained in every structure ofthe pattern from the standard deviation σ and the average gray level A,and the angle θ at which the value of (σ/√A) becomes the minimum,namely, the angle θ of the analyzer in the bright-field optical systemis determined. Alternatively, the angle θ may be determined by settingthe angle of the half-wave plate to θ. The table for the acquired angleθ and the table for the corresponding structure of the pattern areformed and stored in the storage unit. During the inspection, the angleθ corresponding to the pattern that becomes the inspection target isfound out from the table, and the angle of the analyzer (or half-waveplate) is set to the value (θ).

In the case that a bright-field optical system includes a light sourcethat emits the linearly-polarized light, a half-wave plate, anon-polarizing beam splitter, and an analyzer, while the half-wave plateis disposed between the light source and the non-polarizing beamsplitter, the angle of the half-wave plate is desirably set, such thatthe polarization direction of the light with which the substrate isilluminated, at 0 degrees or an angle near 0 degrees, and 90 degrees oran angle near 90 degrees with respect to the pattern formed on thesubstrate, specifically any angle except an angle in the ranges of −5degrees to 5 degrees and 85 degrees to 95 degrees.

On the other hand, for the inspection apparatus provided with thedark-field optical system, the optical image of the substrate differentfrom the substrate that becomes the inspection target is acquired beforethe inspection, and the standard deviation σ of the gray level and theaverage gray level A of the optical image are calculated in everystructure of the pattern. At this point, as one example, the structureof the pattern refers to the pattern pitch, the pattern depth, or theline and space ratio. The angles at which the acquire standard deviationσ becomes the minimum, namely, the angles of the wave plate and analyzerthat are provided in the dark-field optical system are determined. Thetable for the structure of the pattern, and the corresponding angles ofthe wave plate and analyzer is formed and stored in the storage unit ofthe inspection apparatus. During the inspection, the angle correspondingto the pattern that becomes the inspection target is found out from thetable, and the wave plate and analyzer are set to the correspondingangles.

In the method in which the angle θ is obtained in every substrate thatbecomes the inspection target when the electric field amplitude of thescattering light caused by the edge roughness becomes the minimum,because the optimum angle θ of the actual inspection target is obtained,the scattering light caused by the edge roughness can more surely bereduced from the optical image used in the inspection. On the otherhand, in the method for obtaining the optimum angle θ in every structureof the pattern formed on the substrate that becomes the inspectiontarget, the whole inspection time can be shortened because it is notnecessary to obtain the angle θ in every inspection.

An inspection apparatus of the embodiment will be described below.

FIG. 10 is a configuration diagram of an inspection apparatus 100 of thepresent embodiment. The inspection apparatus 100 includes the inspectionoptical system illustrated in FIG. 6. Referring to FIG. 10, the angle ofthe analyzer 1006 can arbitrarily be adjusted by a rotating unit 1006′.The rotating unit 1006′ is controlled by an angle control circuit 14.

As illustrated in FIG. 10, the inspection apparatus 100 includes anoptical image acquiring unit A and a controller B.

The optical image acquiring unit A includes an XY-table 3 that ismovable in a horizontal direction (an X direction and a Y direction), asensor circuit 106, a laser measuring system 122, and an autoloader 130in addition to the inspection optical system described in FIG. 6. TheXY-table 3 may have a structure movable in a rotation direction (θdirection) in addition to the horizontal direction.

A sample 1 that becomes the inspection target is placed on a Z-table 2.The Z-table 2 is provided on the XY-table 3, and is horizontally movabletogether with the XY-table 3. A repetitive pattern such as the line andspace pattern, namely, a regularly repeating pattern having aperiodicity is formed on the sample 1. The template used in thenanoimprint technology can be cited as an example of the sample 1.

Preferably the sample 1 is supported at three points using supportmembers provided in the Z-table 2. In the case that the sample 1 issupported at four points, it is necessary to adjust a height of thesupport member with high accuracy. Unless the height of the supportmember is sufficiently adjusted, there is a risk of deforming the sample1. On the other hand, in the three-point support, the sample 1 can besupported while the deformation of the sample 1 is suppressed to theminimum. The supporting member is configured by using a ballpoint havinga spherical head surface. For example, the two support members in thethree support members are in contact with the sample 1 at two corners,which are not diagonal but adjacent to each other in four corners of thesample 1. The remaining support member in the three support members isdisposed in the region between the two corners at which the two othersupport members are not disposed.

The light source 1002 emits the light to the sample 1 in order toacquire the optical image of the sample 1. A wavelength of the lightemitted from the light source 1002 is at least double the pattern pitch.The inspection apparatus 100 is suitable for the inspection of anultrafine pattern having a line width of 50 to 60 nm or less, andpreferably a light source that emits DUV (Deep UltraViolet radiation)light is used as the light source 1002. The use of the DUV light canrelatively simply configure the optical system, and inspect the finepattern with throughput higher than that of an EB (Electron Beam).

The resolution limit of the optical system in the inspection apparatus100, namely, the resolution limit (R=λ/2 NA) defined by the wavelength(λ) of the light emitted from the light source 1002 and a numericalaperture (NA) of the objective lens 1005 is the value that does notresolve the pattern formed on the sample 1.

In an illumination optical system, the light emitted from the lightsource 1002 is transmitted through the half-wave plate 1003 andreflected by the non-polarizing beam splitter 1004, and the sample 1 isirradiated with the light through the objective lens 1005. The lightreflected by the sample 1 is incident to the image capturing sensor 1007through the objective lens 1005, the non-polarizing beam splitter 1004,and the analyzer 1006. The optical image of the pattern formed on thesample 1 is thus obtained.

The light with which the sample 1 is illuminated is thelinearly-polarized light having the polarization plane of 45 degreeswith respect to the optical axis. Therefore, the difference insensitivity between the large defect such as the short-circuit defect,the open-circuit defect, and the small defect such as the defect causedby the edge roughness can emerge with respect to the electric fieldcomponent of the illumination light.

In the inspection apparatus 100, the imaging optical system includes theanalyzer 1006, which allows only the light having the specificpolarization direction to be extracted. Specifically, the direction ofthe analyzer 1006 is set to the direction orthogonal to the polarizationdirection of the light scattered by the defect of the edge roughness,whereby the scattering light from the defect can almost completely beprevented from being incident to the image capturing sensor 1007. On theother hand, for the light scattered by the short-circuit defect or theopen-circuit defect, because the polarization direction inclines, thelight is incident to the image capturing sensor 1007 through theanalyzer 1006. Accordingly, an optical image in which a short-circuitdefect or an open-circuit defect remains while the defect caused by theedge roughness is removed can be obtained.

The controller B in FIG. 10 will be described below.

In the controller B, a control computer 110 that controls the wholeinspection apparatus 100 is connected to a position circuit 107, animage processing circuit 108, the angle control circuit 14, an patterngenerating circuit 131, a reference image generating circuit 132, acomparing circuit 133, a defect detecting circuit 134, an autoloadercontrol circuit 113, a XY-table control circuit 114 a, a Z-table controlcircuit 114 b, a magnetic disk unit 109, a magnetic tape unit 115, andflexible disk unit 116, which are examples of a storage unit, a display117, a pattern monitor 118, and a printer 119 through a bus 120 thatconstitutes a data transmission line. The image processing circuit 108corresponds to the image processor of the present invention, the defectdetecting circuit 134 corresponds to the defect detector of the presentinvention, and the magnetic disk unit 109 corresponds to the storingunit of the present invention.

The Z-table 2 is driven by a motor 17 b that is controlled by a Z-tablecontrol circuit 114 b. The XY-table 3 is driven by a motor 17 a that iscontrolled by an XY-table control circuit 114 a. A linear motor, as oneexample, can be used as each motor.

In the optical image acquiring unit A in FIG. 10, the image sensor 1007,acquires the optical image of the sample 1. An example of the specificmethod for acquiring the optical image will be described below.

The sample 1 is placed on the vertically movable Z-table 2. The Z-table2 is also horizontally movable by the XY-table 3. The laser measuringsystem 122 measures a moving position of the XY-table 3, and transmitsthe moving position to the position circuit 107. The sample 1 on theXY-table 3 is automatically conveyed from the autoloader 130 that isdriven by the autoloader control circuit 113, and the sample 1 isautomatically discharged after the inspection is ended.

The light source 1002 emits the light with which the sample 1 isirradiated. The light emitted from the light source 1002 is transmittedthrough the half-wave plate 1003, reflected by the non-polarizing beamsplitter 1004, and focused onto the sample 1 through the objective lens1005. Desirably the angle of the half-wave plate 1003 is set such thatthe polarization direction of the light with which the sample 1 isilluminated at 0 degrees or an angle near 0 degrees, and 90 degrees oran angle near 90 degrees with respect to the pattern formed on thesubstrate, specifically any angle except an angle in the ranges of −5degrees to 5 degrees and 85 degrees to 95 degrees. A distance betweenthe objective lens 1005 and the sample 1 is adjusted by perpendicularlymoving the Z-table 2.

The light reflected by the sample 1 is incident to the image capturingsensor 1007 through the objective lens 1005, the non-polarizing beamsplitter 1004, and the analyzer 1006. The optical image of the patternformed on the sample 1 is thus obtained.

FIG. 11 is a view illustrating a procedure to acquire the optical imageof the pattern formed in the sample 1.

As illustrated in FIG. 11, an evaluation region on the sample 1 isvirtually divided into plural strip-like frames 20 ₁, 20 ₂, 20 ₃, 20 ₄,. . . . The XY-table control circuit 114 a controls motion of theXY-table 3 in FIG. 10 such that the frames 20 ₁, 20 ₂, 20 ₃, 20 ₄, . . .are continuously scanned. Specifically, the images having a scan width Was illustrated in FIG. 11 are continuously input to the measuring imagesensor 1007 while the XY-table 3 moves in the X-direction. That is,after the image of the first frame 201 is acquired, the image of thesecond frame 20 ₂ is acquired. In this case, the optical image isacquired while the XY-table 3 moves in the opposite direction to thedirection in which the image of the first frame 20 ₁ is acquired, andthe images having the scan width W are continuously input to each imagesensor. In the case that the image of the third frame 20 ₃ is acquired,the XY-table 3 moves in the opposite direction to the direction in whichthe image of the second frame 20 ₂ is acquired, namely, the direction inwhich the image of the first frame 20 ₁ is acquired. A hatched-lineportion in FIG. 11 schematically expresses the region where the opticalimage is already acquired in the above way.

After the image of the pattern formed the-measuring image sensor 1007illustrated in FIG. 10, is subjected to photoelectric conversion, thesensor circuit 106 performs A/D (analog-digital) conversion to theimage. For example, the line sensor in which CCD cameras that are of theimage acquiring elements are arrayed in line is used as each imagesensor. A TDI (Time Delay Integration) sensor can be cited as an exampleof the line sensor. In this case, the image of the pattern in the sample1 is acquired by the TDI sensor while the XY-table 3 continuously movesin the X-axis direction.

The optical image data to which the sensor circuit 106 performs the A/Dconversion is transmitted to the image processing circuit 108. In theimage processing circuit 108, the optical image data is expressed by thegray level in each pixel. Then, in the image processing circuit 108, thestandard deviation σ of the gray level and the average gray level A areobtained with respect to the acquired optical image, and the value of(σ/√A) is calculated from the standard deviation σ and the average graylevel A.

In the present embodiment, the angle of the analyzer 1006 is changed toacquire plural optical images, and the values of (σ/√A) obtained fromthe optical images are plotted with respect to the angle (θ) of theanalyzer 1006. As described above, the electric field amplitude of thereflected light caused by the edge roughness is proportional to thevalue in which the standard deviation σ of the gray level of the opticalimage is divided by the square root of the average gray level A.Therefore, the angle θ of the analyzer is obtained when the value of(σ/√A) becomes the maximum, thereby determining the condition thatminimizes the bright and dark amplitude caused by the edge roughness.

The information on the angle θ determined by the image processingcircuit 108 is transmitted to the angle control circuit 14. The anglecontrol circuit 14 then controls the angle of the analyzer 1006.Specifically, the angle control circuit 14 controls operation of therotating unit 1006′ such that the angle of the analyzer 1006 becomes theangle θ acquired by the image processing circuit 108.

In the present embodiment, the angle θ of the analyzer is obtained whenthe value of (σ/√A) becomes the minimum. Alternatively, the angle of thehalf-wave plate 1003 may be obtained when the value of (σ/√A) becomesthe minimum. In this case, the half-wave plate 1003 includes a rotatingunit, and the angle control circuit 14 controls the operation of therotating unit, whereby the angle of the half-wave plate 1003 becomes theangle θ acquired by the image processing circuit 108.

By controlling the polarization state of the illumination light and acondition of a polarization control element of the optical systemforming the image of the light reflected from a substrate that becomesan inspection target, the bright and dark unevenness caused by the edgeroughness can be removed with the polarization control element toextract only a change in amplitude of the short-circuit defect oropen-circuit defect. That is, the defect caused by the edge roughness isremoved in the optical image obtained by the image sensor 1007. Asdescribed above, the optical image data is transmitted to the imageprocessing circuit 108 through the sensor circuit 106.

As mentioned above, in the image processing circuit 108, the pixel datain the optical image (in which the defect caused by the edge roughnessis removed) is expressed by the gray level in each pixel. The inspectionregion of the sample 1 is divided into predetermined unit regions, andthe average gray level of each unit region is obtained. For example, thepredetermined unit region may be set to the region of 1 mm×1 mm.

The information on the gray level obtained by the image processingcircuit 108 is transmitted to the defect detecting circuit 134. Forexample, the defect detecting circuit 134 has upper and lower thresholdsaround the average gray level, and has a function of recognizing thegray level as the defect to output the result when the gray levelexceeds the threshold. The threshold level is already predetermined.

As mentioned above, the angle θ is obtained with respect to the sample 1that becomes the inspection target when the electric field amplitude ofthe scattering light caused by the edge roughness becomes the minimum.Alternatively, before the inspection, the angle θ at which the electricfield amplitude of the reflected light caused by the edge roughnessbecomes the minimum may be obtained using a substrate different from thesample 1. The table for the structure of the pattern formed on thesubstrate and the corresponding optimum angle θ is formed and stored inthe magnetic disk unit 109. During the inspection, the table suitable tothe substrate that becomes the inspection target is read from themagnetic disk unit 109, and the angle control circuit 14 controls theoperation of the rotating unit 1006′ such that the analyzer 1006 becomesthe optimum angle according to the pattern.

The inspection apparatus of the embodiment can also have a reviewfunction in addition to the inspection function. As used herein, thereview means an operation in which an operator determines whether thedetected defect becomes a problem.

For example, a coordinate of a point, which is determined to be thedefect by the comparing circuit 133 in FIG. 10, and the optical imageand a reference image, which become a basis of the defect determination,are transmitted to a review unit (not illustrated). The operatorperforms the review by comparing the reference image that becomes thebasis of the defect determination to the optical image including thedefect. Specifically, the image of the defect point of the sample 1 isdisplayed using the optical system illustrated in FIG. 10. At the sametime, the judgment condition of the defect determination, and theoptical image and a reference image, which become the basis of thedefect determination, are displayed on a screen of the control computer110. The defect information obtained by the review is stored in themagnetic disk unit 109.

When at least one defect to be corrected is recognized by the review,the sample 1 and a defect information list are transmitted to acorrection apparatus (not illustrated) that is of an external apparatusof the inspection apparatus 100. Because a correction method depends onwhether a type of defect is a convex defect or a concave defect, thetype of the defect including the differentiation between the convexdefect and the concave defect and the coordinate of the defect are addedto the defect information list.

In the above example, the line and space pattern is cited as therepetitive pattern. However, the embodiment is not limited to the lineand space pattern. The embodiment can be applied to the repetitivepatterns such as a hole pattern as long as the repetitive pattern hasthe period at the resolution limit or less.

As described above, according to the inspection apparatus of theembodiment, even in the sample in which the repetitive pattern havingthe period at the resolution of the optical system or less is formed,the inspection can be performed while the defect to be detected andotherwise are distinguished from each other. Specifically, thescattering component caused by the edge roughness can be removed, anddistinguished from the scattering component caused by the short-circuitdefect or the open-circuit defect.

The light source that emits the DUV (Deep UltraViolet radiation) lightcan be used in the inspection apparatus of the embodiment. Therefore,the inspection can be performed without generating the throughputdegradation, which becomes a problem in the case that the EB (ElectronBeam) is used as the light source.

The present invention is not limited to the embodiments described andcan be implemented in various ways without departing from the spirit ofthe invention.

For example, the inspection apparatus according to the presentembodiment as shown in FIG. 10, includes the optical system as shown inFIG. 6, and can further include the optical system as shown in FIG. 7.That is, the inspection apparatus 100 as shown in FIG. 10, includes thenon-polarizing beam splitter 1004, and can further include a polarizingbeam splitter, a quarter-wave plate, and a half-wave plate. In thiscase, the optical image in which the reflected light caused by the edgeroughness is removed can be obtained by adjusting the angle of thehalf-wave plate 3004 a.

In the case that the optical system in FIG. 7 is used in the inspectionapparatus of the present embodiment, the plane of light incidence withrespect to the polarization beam splitter 3006 in FIG. 7 is matched withthe direction of the repetitive pattern formed in the mask 3001, thedirection of the optical axis of the half-wave plate 3004 a is set to 23degrees with respect to the direction of the repetitive pattern, and thedirection of the optical axis of the quarter-wave plate 3004 b is set to45 degrees with respect to the direction of the repetitive pattern.Assuming that the light is not influenced by the repetitive pattern, thepolarization direction of the linearly-polarized light, which is emittedfrom the light source 3002 and reflected by the polarization beamsplitter 3006, is identical to the direction (that is, 0 degrees) of therepetitive pattern formed in the inspection target (in FIG. 7, mask3001), and the linearly-polarized light becomes the linearly-polarizedlight having the polarization direction of 46 degrees with respect tothe direction of the repetitive pattern when being transmitted throughthe half-wave plate 3004 a. The linearly-polarized light then becomesthe elliptically-polarized light having the polarization direction of 45degrees with respect to the direction of the repetitive pattern whenbeing transmitted through the quarter-wave plate 3004 b. Theelliptically-polarized light is reflected by the inspection target,transmitted through the quarter-wave plate 3004 b, and becomes thelinearly-polarized light having the polarization direction of 44 degreeswith respect to the direction of the repetitive pattern. Then, thelinearly-polarized light is transmitted through the half-wave plate 3004a to become the linearly-polarized light having the polarizationdirection of 2 degrees with respect to the direction of the repetitivepattern, and the linearly-polarized light is incident on thepolarization beam splitter 3006.

In the optical system, the polarization direction of the light incidentto the polarization beam splitter 3006 is rotated by the rotation of thehalf-wave plate, so that the polarization direction of the lightreflected by the edge roughness can be set to 45 degrees. That is, whenthe reflected light caused by the edge roughness has the polarizationdirection of 43 degrees, the reflected light can be rotated by 2 degreesand set to 45 degrees. Therefore, the reflected light caused by the edgeroughness can completely be prevented from being transmitted through theimage capturing sensor 3007.

The above description of the present embodiment has not specifiedapparatus constructions, control methods, etc. which are not essentialto the description of the invention, since any suitable apparatusconstructions, control methods, etc. can be employed to implement theinvention. Further, the scope of this invention encompasses allinspection apparatuses employing the elements of the invention andvariations thereof, which can be designed by those skilled in the art.

1. (canceled)
 2. An inspection apparatus comprising: an illuminationoptical system including a light source that emits linearly-polarizedlight having a predetermined wavelength, the illumination optical systemreflecting the linearly-polarized light emitted from the light sourcethrough a half-wave plate using an non-polarizing beam splitter, formingthe linearly-polarized light into the linearly-polarized light having apolarization plane of any angle except an angle in ranges of −5 degreesto 5 degrees and 85 degrees to 95 degrees with respect to a repetitivedirection of a repetitive pattern formed on a sample that is aninspection target, and illuminating the sample; an imaging opticalsystem including an image capturing sensor that obtains an optical imageof the pattern formed on the sample, the imaging optical systemtransmitting the light reflected by the sample through a lens, thenon-polarizing beam splitter, and an analyzer, and forming an image ofthe light on the image capturing sensor; an image processor that obtainsan average gray level and a standard deviation in every predeterminedunit region in the optical image; and a defect detector that detects adefect of the sample, wherein a resolution limit defined by a wavelengthof the light source and a numerical aperture of the lens is a value inwhich the pattern is not resolved, and the image processor obtains anangle of the analyzer or the half-wave plate from the plurality ofoptical images acquired by changing the angle of the analyzer or thehalf-wave plate such that a value of (σ/√A) obtained from a standarddeviation σ of a gray level and an average gray level A becomes aminimum.
 3. The inspection apparatus according to claim 2, wherein theangle of the analyzer or the half-wave plate can arbitrarily be adjustedusing a rotating unit, and the angle of the analyzer or the half-waveplate is controlled such that the value of (σ/√A) acquired by the imageprocessor becomes the minimum.
 4. The inspection apparatus according toclaim 2, comprising a storage unit in which information from the imageprocessor is stored, wherein, with respect to a substrate in which aplurality of patterns having different structures are formed or aplurality of substrates in which single patterns having differentstructures are formed, the image processor acquires the angle of theanalyzer or the half-wave plate in every structure of the pattern fromthe plurality of optical images acquired by changing the angle of theanalyzer or the half-wave plate such that the value of (σ/√A) obtainedfrom the standard deviation σ of the gray level and the average graylevel A becomes the minimum, and the image processor transmits theacquired-angle of the analyzer or the half-wave plate to the storageunit, and based on information stored in the storage unit, the angle ofthe analyzer or the half-wave plate is controlled to acquire the opticalimage of the pattern formed on the sample.
 5. The inspection apparatusaccording to claim 4, wherein the angle of the analyzer or the half-waveplate can arbitrarily be adjusted using a rotating unit, and operationof the rotating unit is controlled based on the information stored inthe storage unit.